[Security] Added a REMOTE_USER based listener to security firewalls

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | /
| License       | MIT
| Doc PR        | symfony/symfony-docs#3912
TODO 
- [x] submit changes to the documentation

I've seen myself implementing a few times a REMOTE_USER based authentication listener, as a large part of security modules for Apache (Kerberos, CAS, and more) are providing the username via an environment variable.

So I thought this could benefit the whole community if directly included in the framework. It is very similar to the X509AuthenticationListener, and basing the RemoteUserAuthenticationListener on the AbstractPreAuthenticatedListener is relevant and very convenient.

Using the X509AuthenticationListener could be possible, but it is confusing to use it directly when your authentication is not certificate based.

Please let me know if I need to update anything.

Regards